BTW, this was specified in 821 because it had been a factor in earlier experience of the ARPANET [733 and using FTP as a mail transport, which was how much of this all started], and binary was explicitly not allowed. 822 force 7-bit ASCII because of the earlier issues of things like CDC's display code (what a nightmare), much less EBCDIC. The key was that those of us in the ARPANET community could not allow "anything," - but we did have to detail what was there.
Some of us lived in this world and wrote programs that dealt with those constraints at the time. I am relaying to you what it was and how it happened. As I said, we have what we have because that was what we were living with -- two distinct worlds, the ARPANET community - which was setting the standards for interchange, and UNIX (USENET), which was de facto and growing because it cost little to join it.
As a remark, the MBOX format as standardized by POSIX decades ago
Ouch -- I was part of POSIX and /usr/group before that. And had >>nothing<< to do with it. The POSIX definition was at least 15 years After Bruce wrote MH and Kurt did delivermail and if I count I >>suspect<< it is correctly closer to 20-25. mbox was created I believe by Ken (Doug do you remember?), but I'm not sure who actually wrote the original "mail" program for Fifth edition (maybe Fourth) - which as I said was both an MUI and a MTA. But that development was over 15 yrs before we started the /usr/group standard, much less the POSIX ones.
You are probably correct that until it was formally specified in the POSIX definition, the format was defined originally in Ken's code, then Kurt's and finally in Eric's. IIRC, Bruce actually had a man page in MH that described his format that used the ^A characters, but I would have to rummage through old sources to be 100% certain. Certainly, later distributions of it did describe it, and MMDF may have also - but I'm not sure.
BTW: by about 4.2BSD time (maybe a little earlier), particularly because of sendmail - the MH system (which had left Rand and was then being supported by someone else ??UC Irvine maybe??) had been hacked to handle the mbox format